The invention relates to variable valve timing mechanisms and, more particularly, to valve actuating mechanisms for varying the lift and timing of engine valves.
U.S. Pat. No. 5,937,809, issued Aug. 17, 1999, discloses cam driven variable valve timing (VVT) mechanisms which are relatively compact, and are applicable for operating individual or multiple valves. In these mechanisms, an engine valve is driven by an oscillating rocker cam that is actuated by a linkage driven by a rotary eccentric, preferably a rotary cam. The linkage is pivoted on a control member that is, in turn, pivotable about the axis of the rotary cam and angularly adjustable to vary the orientation of the rocker cam and thereby vary the valve lift and timing. The rotary cam may be carried on a camshaft. The oscillating cam is pivoted on the rotational axis of the rotary cam.
U.S. patent application Ser. No. 09/129,270, filed Aug. 5, 1998, now 6,019,076, discloses a similar cam actuated VVT mechanism having various additional features, including a variable ratio pin and slot control member drive providing advantageous control characteristics and a worm drive for the control shaft designed to prevent backdrive forces from overcoming the actuating force of the small drive motor. A particular embodiment of flat spiral mechanism return springs is also disclosed.
The present invention provides crank driven VVT mechanisms wherein single or dual cranks are provided for actuating the oscillating cam drive mechanisms. The crank drive is desmodromic in that it actuates the mechanisms in both valve opening and valve closing directions and thus avoids the need to provide return springs as are generally required in cam driven mechanisms to bias the mechanisms toward a valve closed position. However, the crank driven mechanisms of the invention require the oscillating cams to pivot onto the base circle portion during a dwell period in order to provide periods of valve closed engine operation even when the valves are set for maximum opening stroke. Thus, increased motion of the actuating mechanism or a smaller angular extent of the valve lift portions of the oscillating cams is required as compared to a cam driven mechanism.
The advantages of control by a variable ratio slide and slot control lever drive as well as a back force limiting worm drive for the control shaft may be combined with the crank mechanism to provide additional system advantages comparable to those of cam actuated mechanisms.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.